Aquaculture nets or fish-farming nets are used to raise aquatic life such as fish. The aquaculture net keeps the aquatic life controlled and contained and protects the aquatic life inside the net against predators such as sharks, sea lions or sea wolfs.
The dimensions of such an aquaculture net are considerable. An example of a typical dimension is 30 m×30 m×15 m, the last dimension being the depth of the net inside the water and the first two dimensions being the width and length of the net at the water surface. The net may be formed of a polymer wire or of a coated steel wire. As a matter of example only, a net made of galvanized steel wire and of the above-mentioned dimensions has a weight above 4 metric Tons.
The aquaculture nets are usually of the chain link fence type. This is a fence of steel wires woven into a diamond pattern. The meshes have a dimension that is smaller than the dimension of the fish contained in the nets. Each steel wire is preformed by bending so that it exhibits a wavy pattern with maxima and minima. The maxima of a steel wire interlock with the minima of a neighboring wire to form the patterns of a series of diamonds.
Aquaculture nets of the chain link fence type have proven to be successful to control the aquatic life and to protect against predators. Aquaculture nets with galvanized steel wires offer an acceptable resistance against bio-fouling, i.e. against fouling material that may grow on the mesh structure. Within the context of the present invention, the terms fouling material refer to fouling organisms such as barnacles, algae or molluscs, which may attach and grow to the wire material of the mesh structure. This fouling mechanism is so persistent that entire openings in the meshes may be filled blocking any introduction of fresh water or nutrition into the volume inside the mesh structure.
A main part of the aquaculture nets have circular rafts, i.e. the top of the aquaculture net has the form of a circle. Fish and predator nets are hanging on these rafts and must provide the required volume for fish growing. An existing embodiment (see FIG. 1) provides an aquaculture net 10 with a circular top 12 and a square bottom (cube) cage 14. The side walls 16 connect the square bottom 14 to the top 12. Although wide spread in use, such a cage however has two main disadvantages.
A first disadvantage is the reduction of the contained volume with 11%. Take as a matter of example, a 30 m diameter cage with a 94.2 m perimeter and an area of 707 m2. To fit a square cage in the same perimeter, the side of the square must be of 23.6 m length, with an area of 555 m2. By having a cage with square bottom and circular upper section, the mean or average area is 613 m2, corresponding to 89% of the area of the circular upper section. Thus the inner volume of the aquaculture net will also be reduced to the same degree, i.e. 11%. This 11% loss in volume may be compensated by increasing the net depth, however leading to a weight increase of the net. This weight increase is preferably avoided, since it may drastically reduce the life time of the aquaculture net due to the fact that the upper wires constituting net have the carry the total weight of the net and are subjected to severe external conditions such as the tidings in an aggressive environment as sea water.
A second disadvantage is the uneven load (weight) distribution applied to the upper circular part of the raft. The load applied in a point corresponding to a corner of the square bottom is 25% under the mean weight, while the load applied in a point corresponding to the middle of each square's side is 25% above the mean weight. Thus, the raft tends to deform and may break under severe conditions. The uneven load may be partly solved by adding extra counterweights in determined points of the net. Again, however, this solution increases the total weight of the net, which is to be avoided because of the reasons mentioned above.